“MEMS” generally refers to an apparatus incorporating some mechanical structure having a dimensional scale that is comparable to microelectronic devices, for example, less than approximately 250 um. This mechanical structure is typically capable of some form of mechanical motion and is formed at the micro-scale using fabrication techniques similar to those utilized in the microelectronic industry, such as thin film deposition, and thin film patterning by photolithography and reactive ion etching (RIE). The micromechanical structure in a MEMS device distinguishes a MEMS device from a microelectronic device.
Certain MEMS devices include a resonator. MEMS resonators are of particular interest in timing devices for an integrated circuit (IC). The resonator may have a variety of physical shapes, such as, but not limited to, beams and plates. FIG. 1 depicts a conventional MEMS device 100 including a resonator 105 coupled to a substrate 102 via an anchor 104. During operation resonator 105 is electrostatically driven by a first electrode 110 to dynamically deflect so as to increase its capacitance by decreasing its gap from electrode 110 when a voltage differential exists between the two. Because electrode 110 and resonator 105 are the same height and in the same plane as the structural layer, the resonator 105, when driven, deforms laterally across a distance between electrode 110 and a second electrode 111, remaining in a plane 103 of the electrode 110. The plane 103 is parallel to substrate 102 and therefore defines the layout area or “footprint” of conventional MEMS device 100. Electrode 111 detects the resonant frequency of resonator 105 as the capacitance varies between the two in response to the deflection driven by electrode 110. Because the resonator is driven to resonate in a mode where the resonator 105 remains in the plane 103 of the electrode 110, the conventional MEMS device 100 is commonly referred to as an “in-plane” or “lateral” mode resonator.
There are several drawbacks to the parallel-plate-capacitor drive and sense mechanism of conventional MEMS device 100. The electrostatic force is nonlinear unless the amplitude of vibration is limited to a small fraction of the capacitor gap. The quality factor Q of the resonance may be limited by squeeze-film dampening. Furthermore, because the transduction efficiency of resonator 105 is dependent on the area of the parallel-plate capacitor formed between the resonator 105 and electrode 110, fabrication of the resonator 105 generally includes a number of techniques to ensure the resonator 105, when released, remains perfectly flat and in the plane of the electrode 110. Such fabrication techniques are often thermally taxing or require prohibitively expensive or commercially unfeasible methods.